The present invention relates to superconducting quantum interference devices, hereinafter referred to as SQUID's, and more particularly to SQUID type magnetometer and gradiometer arrays for measuring components of a magnetic field vector and components of the magnetic field gradient tensor.
The SQUID is a circuit device which is designed to be operated at extremely low temperatures. The SQUID utilizes materials which, when refrigerated to temperatures typically 4 to 10 degrees above absolute zero temperature, become superconducting and exhibit zero resistance. At such low temperatures and below a critical magnetic field strength, a superconductor establishes currents in a thin surface layer which excludes the applied magnetic field from all but a thin surface layer of the superconductor's interior. This magnetic flux exclusion is known as the Meissner effect, and it is a property which is useful in the design and development of magnetometers.
When using SQUID's as sensors for measuring various quantities, such as magnetic field and magnetic field gradients, two superconductors are separated by a very thin insulator to form a structure commonly known as a Josephson junction. A small "supercurrent" can be made to flow across the "insulated" gap between the two films with zero resistance and no voltage, just as if it were a single piece of superconductor. This flow of current is a case of quantum mechanical tunneling of superconducting electron pairs through the insulator giving rise to what is known as Josephson tunneling. Other superconducting junction device configurations may be used in the SQUID in place of the above-described "insulated" junction. For example, the two superconductors may be separated by a thin semiconductor, or by a thin normal metal, or by a very narrow superconducting bridge.
If the tunneling current in a Josephson junction exceeds a critical value, a voltage will appear across the junction and electromagnetic radiation will be emitted. This phenomenon is called the AC-Josephson effect. When the Josephson-junction device is incorporated in a superconductive loop, the most sensitive gauge known for measuring magnetism is created. As the strength of ambient magnetic field changes, the rf impedance of the circuit also changes. By measuring changes in this impedance, the strength of the magnetic field can be determined. This effect is produced by quantum mechanical interference between electrons traveling around the loop, such effect giving rise to the acronym "SQUID".
In the field of cryogenic magnetometry, it has been the general practice to employ SQUID magnetometers to measure components of a magnetic field. Likewise, SQUID gradiometers are typically used to measure a magnetic field gradient (i.e., the spatial variation of the field) by measuring simultaneously several spatial derivatives of a magnetic field. This characterizes the magnetic environment and provides data for calculation of the range, bearing and magnetic moment of any magnetic irregularity to the extent that it can be described as a magnetic dipole. Complete magnetic field vector data and complete gradient tensor data may also allow determination of range and bearing for magnetic irregularities which are more complex than simple dipoles. In any event, the present invention will facilitate magnetic surveys both for man-made objects and interesting geological and natural resource features.
In current practice, if it is desired to measure more than one component of the magnetic field vector, additional SQUID magnetometers must be used, i.e., each magnetometer detects only one vector field component. Similarly, a separate SQUID gradiometer is required to measure each independent element of the gradient tensor. Typically, a plurality of magnetometers and gradiometers are used to measure and characterize a magnetic field. For example, in IEEE Transaction of Magnetics, Volume MAG-II, Number 2, pages 701-707, March 1975, a system is described which utilizes three stationary SQUID magnetometers and five stationary SQUID gradiometers to determine all three vector field components and the five independent elements of the gradient tensor. The construction and operation of such an extensive array of magnetometers and gradiometers is very complex, time consuming, and difficult in part due to the precise requirements of balancing the gradiometer loops. Specifically, in order to assure reliable results with gradiometers, it is necessary to balance accurately the effective areas of the opposing sensing loops, as well as to obtain an exact angular relationship between them. Any variation from alignment and identical areas results in "sense loop imbalance" which gives rise to a sense loop signal proportional to field fluctuations and leads to an increased noise level. Each gradiometer constitutes a potential independent source of magnetic noise which can interfere with the signals generated by the SQUID sensors. The greater the number of gradiometers used in an array, the more difficult the balancing task becomes. A commonly used balancing technique makes use of small superconducting vanes which may be positioned so as to vary the distribution of magnetic flux intercepted by the gradiometer loops.